An optical probe is a measuring device, which can be used like a traditional Coordinate Measuring Machine (CMM) arm supplied by manufacturers such as Faro or Romer. However, unlike a CMM arm, an optical probe has no mechanical mechanisms to wear, brake or calibrate. Instead, it consists of a hand held device with markers, a digital camera and a computer. The digital camera images the hand held device and its markers and a computer calculates the position of the hand held device from the position of the markers in the digital image. This allows the hand held device to be used to take measurements
While other optical probes do exist they suffer from a number of problems. One group of prior art probes requires the use of expensive narrow spectrum cameras and markers. When a digital image of an optical probe is taken, the location of the markers in the image must be determined very accurately, usually to sub pixel level. To achieve a fast measuring speed or high frame rate, it is essential that the markers are extracted quickly from the digital images, e.g., in a time of approximately 30 milliseconds. If the image contains a lot of data other than the markers, a large amount of image processing is required to extract the markers. To reduce unwanted image content, the prior art systems use narrow spectrum digital cameras (e.g., infra-red or near infra-red) and point light source markers. These cameras and markers are much more specialized and consequently more expensive than traditional visible light cameras.
These systems also require the use of point light source markers and these point light source markers can introduce inaccuracy into the system. The point light source markers facilitate fast extraction of the markers from the image as these require very little image processing. However, point light sources can create systematic errors in their estimated location when they are either overexposed, moving, out of focus and affected by the inevitable perspective associated with any image.
Other systems use high frequency digital image acquisition techniques often referred to as an electronic shutter. As an optical probe is usually a hand held device, it cannot be considered a static device. If the probe moves at a speed of 100 mm/s, then it will move 0.1 mm in 1 millisecond. The accuracy of a typical CMM arm is less than 0.1 mm, therefore it is essential that the optical probe is imaged in as short a time as possible. In order to achieve this, prior art optical probes electronically read the digital camera sensor at a very high frequency. In the case of CCD sensors, this digital process is prone to blooming, smearing or overexposure during the imaging process, and in the case of CMOS sensors, this allows tearing of the image. While both of these problems may be acceptable in a traditional imaging system, they introduce unwanted errors in the estimation of the markers location.